Grain sorghum varietal reactions to heat stress and environment
1 The University of Queensland, Centre for Plant Science, QAAFI, Brisbane
2 CSIRO Plant Industry, Brisbane
3 Agri-Science Queensland, DEEDI, Toowoomba
4 The University of Queensland, Centre for Plant Science, QAAFI, Warwick
Take home messages
- Sorghum seed set is reduced by high temperature effects (>36-38oC) on pollen around flowering
- Sorghum genotypes differ in their tolerance to high temperature stress
- Risk of high temperature damage depends on sowing date and variety
- Climate change will exacerbate high temperature effects but avoidance by crop management and genetic tolerance seems possible
- While late plantings (mid Jan) in NNSW avoid heat, late cultivars sown at this time have increased risk of cold conditions that reduce grain yield, and exposure to weather that may favour ergot.
Varietal attributes, such as heat stress tolerance, tillering, and maturity can all have large effects on yield. However, this will depend on starting soil water, time of sowing, crop management, and the nature of the season.
There are two things that are for sure –
- we do not know what the yield outcome will be at the time of sowing, and
- the superiority of specific genotype and management combinations varies from year-to-year depending on how the season transpires.
The best we can do is to estimate the risks of what might happen for different scenarios given historical climate data. The APSIM model provides the best technology for doing this and the sorghum model has been recently updated to incorporate the latest scientific knowledge on the physiology of crop growth and development (Hammer et al., 2010). We can now simulate risks associated with changes in genetics (G) and management (M) across environments (E) – the G*M*E landscape - with increased confidence by using the model with historical climate records (or with climate change scenarios).
Here we look at specific varietal attributes associated with heat stress tolerance and how they might affect yield outcomes. Recent research on high temperature tolerance is summarised and its implications for production risks evaluated by a crop simulation analysis.
For N NSW, late plantings can also risk exposure to low temperatures that affect pollen fertility and can favour the development of ergot.
High temperature effects on sorghum
We have recently conducted a range of controlled environment and field experiments to study the physiology and genetics of high temperature effects on growth and development of sorghum.
- Controlled environment experiment on varietal reactions to high temperature stress - 24 diverse genotypes were grown without water limitation at four day/night temperatures ranging from of 30/22° (standard) to 38/22° (high) for their whole life cycle.
- Controlled environment experiment on timing of high temperature effect - a tolerant and susceptible genotype were grown under either standard or high temperature for their whole life cycle except for 5 days when they were trnsferred to the other temperature
- Field experiment to validate effects in controlled environment – a selection of genotypes were grown in the field either with/without specifically designed covers that raised daytime maximum temperature by about 6°C.
High temperature conditions affected both vegetative and reproductive growth of the sorghum genotypes. High temperature increased development rate (i.e. shorter time to flowering), leaf number, and leaf appearance rate, but had no effect on leaf size. However, there was significant reduction in plant height, pollen viability and seed set under high temperature (Fig .1). There was significant variability in seed set and pollen viability responses among sorghum genotypes (Fig. 2) (Singh et al., 2015). The most tolerant genotypes showed only small reduction in seed set at 38°C, whereas the most susceptible showed significant reductions at 36°C. Seed set was highly correlated with pollen viability. All treatments were well-watered so this effect of high temperature on seed set is independent of the effect of moisture stress.
Figure 1. Effect of high temperature on seed set of B923296 (left panel) and contrasting effect for genotype 85G56 (right panel)
Figure 2. Effect of a range of high temperature treatments on seed set percentage for a diverse set of sorghum genotypes.
This evidence and that from other studies (Prasad et al., 2006) indicated that the effect of high temperature on seed set was most likely related to effects on pollen and thus likely occurred during the period of pollen formation or pollination. This was confirmed in the temperature switching experiment, which showed the effect occurred over about a 10-day period centred on flowering.
Figure 3. Effect on seed set of moving a susceptible cultivar from either Optimum Temperature to High Temperature (OT-HT) or the reverse (HT-OT) for 5-day intervals commencing prior to flag leaf full expansion. The control represents results without transfer.
We used this information to develop an index of the effects of genotype and high temperature on seed set that could be implemented in simulation studies.
Low temperature effects on fertility
There is little recent data on the effects of low temperature on sterility. An older paper on this topic found that during the period from a little before flag leaf appearance to grain set, multiple days of temperatures of less than 13°C would reduce seed fertility (Fig. 4). The effect comes in after 4 to 5 days of less than 13°C and more than two weeks of these conditions will reduce fertility to near zero.
Figure 4. Effect on seed fertility of low night temperatures (from Downes and Marshall 1971).
Implications of varietal reactions to heat stress on yield
We simulated a range of sowing dates (Sept-Jan) at Moree for a standard medium-late maturity sorghum hybrid grown in 1m rows at 50,000 plants/ha on a 150cm deep grey clay soil with water-holding capacity of 285 mm. 50 years of historical climate data (from 1960) was used to simulate yield of crops assuming the same sowing date and starting soil water each year so that the only variable was the seasonal weather (Fig. 5). Simulations were conducted assuming that the profile held 100mm available water at sowing. In the initial set of simulations, the index representing the effect of high temperature on seed set was not invoked.
Figure 5. Yield (kg/ha) likelihood versus sowing date at Moree assuming 100mm available water at sowing. The blue line joins the median (50/50) yield and the bars indicate the range of yield in 80% of years. The best and worst 10% are not included. The 80% of years are broken into the best 20% (top-green), middle 40% (yellow), and worst 20% (bottom-red) of those years.
These results reflect the seasonal patterns of water availability to the crop via rainfall and evaporative demand patterns. With 100 mm water availability at sowing, in-crop rainfall and evaporative demand becomes important. While there is not a large effect of sowing date on yield likelihood, later sowings (Dec) gain a slight advantage at the median. However, these effects must also be considered in conjunction with timing of planting opportunities and cropping system issues.
How are these yield likelihoods affected by incidence of high temperature conditions? The historical climate data was first analysed to examine the frequency and timing of high temperature events in relation to the crop cycle associated with different sowing dates. Then simulations were repeated with the indices of the effect of high temperature on seed set invoked to estimate effects on yield.
Figure 6. Probability of occurrence of either one or four days with a maximum temperature exceeding 36 or 38˚C within an interval of 200˚Cd (10-14 days) commencing at various times of year for key locations in the sorghum cropping region of NE Australia.
The frequency of occurrence in any 10-14 day period of days with maximum temperature >38C is greatest from mid-December until mid- January (Fig. 6). While there are many occasions at Moree (approx 50%) with at least one such day in the 10-14 day period, there are few (approx 10%) with more than 4 days. The effect on pollen viability, seed set, and hence yield, depends on the magnitude of the temperature event and its duration during the critical developmental window of this duration (10-14 days) around flowering.
At Moree, sowings during October will reach flowering at the high risk period for high temperature incidence (data not shown). There will remain some, but lesser, risk for earlier and later sowings. However, as high temperature occurrences can occur over a wide time frame during summer, other sowing times are not totally immune.
The simulated yield reductions due to high temperature effects on pollen viability and seed set varied considerably from year-to-year for a 1 Oct sowing at Moree with a susceptible genotype (Fig. 7). There were many years with more than 10% yield reduction (i.e. relative yield < 0.9), and 8 years out of 50 had severe effects (i.e. relative yield < 0.7). However, for a heat tolerant genotype there were no years with more than 10% yield reduction.
Figure 7. Relative reduction in simulated yield due to high temperature effects for 1 October sowing at Moree with 100mm available soil water with either a susceptible (Group 1 - left panel) or tolerant (Group IV- right panel) genotype
It is plausible to reduce high temperature risks through changing sowing date. At Moree, early (Sept) and late (Nov-Dec) sowings have lower risk on average than sowings in Oct-Nov (Fig. 8), but there are instances of severe effects (i.e. relative yield < 0.7) for all sowing times, except very late sowing (Jan). Hence, the simulations suggest management of sowing time to be a much less effective strategy in moderating high temperature risks than genetic modification. It is also considerably more difficult to implement as there is limited opportunity to control sowing time and cropping system issues (e.g. weed and disease control, rotation opportunities) restrict options.
Figure 8. Relative reduction in simulated yield due to high temperature versus sowing date at Moree for a susceptible sorghum genotype. Each point represents the relative yield for one year of the 50-year simulation and the line connects the median relative yield for each simulated sowing date.
A final point to consider for Moree and other locations in N NSW, is that late summer plantings have potentially higher exposure to cold (Fig. 9). For a late maturing cultivar, planted Jan 15, the median anthesis date is about 79 days after sowing, (1st week of April), and in 1 in 4 years, this cultivar would be exposed to a damaging level of low temperature (5 days < 13°C) which could kill sufficient pollen to reduce yields by about 25% - similar to the heat effect for October plantings (Fig. 9a). For both dates, medium maturing cultivars would avoid these conditions in most years (< 10% risk). However, late maturing cultivars would be at risk of about 25% grain loss due to pollen sterility in 1 in 5 years for 1 Jan sowing and 1 in 2 years for a 15 Jan sowing, i.e. late planting of a late cultivar in this region carries about the same risk of similar yield loss (due to cold) as does an Oct planting due to heat.
In wetter years, ergot risks would also be increased as they are also associated with these levels of cooler temperatures.
Figure 9. (a) Risks of greater than 1 or 5 days with Tmin < 13°C for any time of year in Moree with vertical lines indicating the median anthesis date of a medium or late maturing cultivar sown 15 Jan. (b) The number of days with Tmin < 13°C for a medium or late cultivar sown 1 Jan or 15 Jan.
The heat tolerance issue will be exacerbated as temperatures rise with climate change. The frequencies and severity of high temperature events are predicted to increase over the next decades (IPCC, 2007). It is clear that risks of yield reduction due to high temperature effects will increase. Our studies to date have identified potential sources of genetic tolerance to high temperature effects so that breeding options will be possible. However, it will be important to progress this work to install this level of genetic tolerance into elite sorghum germplasm, while seeking new sources in order to keep ahead of climate change. Interactions with management options, such as sowing date, will become more critical as risks increase.
Downes RW and Marshall DR (1971) Low temperature induced male sterility in Sorghum bicolor. Aust. J. Exp. Agric. & Animal Hus. 11: 352-356
Hammer, G.L., van Oosterom, E., McLean, G., Chapman, S.C., Broad, I., Harland, P. and Muchow, R.C. (2010). Adapting APSIM to model the physiology and genetics of complex adaptive traits in field crops. Journal of Experimental Botany, 61:2185-2202.
IPCC Intergovernmental Panel on Climate Change (2007) ‘Fourth assessment report. Synthesis report.’ (World Meteorological Organization: Geneva)
Prasad PVV, Boote KJ, Allen LH, and Thomas JMG (2006) Adverse high temperature effects on pollen viability, seed-set, seed yield and harvest index of grain sorghum (Sorghum bicolor L.). Agricultural and Forest Meteorology, 139, 237-251.
Singh, V., Nguyen, C.T., van Oosterom, E.J., Chapman, S.C., Jordan, D.R., and Hammer, G.L. (2015). Sorghum genotypes differ in high temperature responses for seed set. Field Crops Research 171: 32-40.
The modelling undertaken as part of this project is made possible by the significant contributions of growers through the support of the GRDC, the authors would like to thank them for their continued support.
The biological research on high temperature effects was conducted as part of a Federal Government Department of Agriculture Forestry and Fisheries funded project on Climate Ready Cereals.
The University of Queensland
Ph: 07 3346 9463
GRDC Project code: UQ00065
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